The present invention relates to a color cathode ray tube having a shadow mask.
In general, a color cathode ray tube has an envelope having a panel and a funnel. The panel includes a substantially rectangular effective portion and a skirt portion provided at a peripheral portion of the effective portion. The funnel is joined to the skirt portion. A phosphor screen is formed on the inner surface of the effective portion of the panel, and comprises three-color phosphor layers which emit three color light components, blue, green, and red. A substantially rectangular shadow mask is arranged inward of and opposite to the panel.
Furthermore, in the color cathode ray tube, an electron gun for emitting three electron beams is provided within a neck of the funnel. The three electron beams emitted from the electron gun are deflected by a deflecting unit mounted on the outer side of the funnel, and horizontally and vertically scanned over the phosphor screen through the shadow mask, thereby displaying a color image.
The shadow mask is provided to sort the three electron beams emitted from the electron gun so that they correctly land on the three-color phosphor layers. The shadow mask comprises a substantially rectangular mask body and a substantially rectangular mask frame having long and short side walls attached to a peripheral portion of the mask body. The mask body includes a surface opposing the phosphor screen and having a number of electron beam passage apertures.
The shadow mask is supported inside the panel. To be more specific, elastic supporting members are fixed to the long and short side walls of the mask frame, and are respectively engaged with supporting pins provided at the skirt portion of the panel, thereby supporting the shadow mask. The supporting members are formed such that pushing pressures of the supporting members, which are caused by the elastic deformation of the supporting members when they are engaged with the supporting pins, equally act on the long and short side walls of the mask frame.
In order for such a color cathode ray tube to display an image having a high color purity on the phosphor screen, the shadow mask needs to be located to have a predetermined positional relationship with the phosphor screen, and in particular, it needs to be positioned such that the distance between the mask body and the inner surface of the effective portion of the panel falls within an allowable range.
However, only about 20% of the electron beams emitted from the electron gun reach the phosphor screen through the electron beam passage apertures of the mask body, and about 80% of the emitted electron beams impinge on the shadow mask. Due to impingement of electron beams, the shadow mask is heated, and thus thermally expands. As a result, it is displaced from the predetermined position, and the electron beams do not correctly land on target phosphor layers, thus lowering the color purity.
More specifically, lowering of the color purity due to thermal expansion of the shadow mask occurs in the following manners:
First, the mask body is mainly heated at an initial stage of the operation of the color cathode ray tube, and expands toward the phosphor screen (doming). Alternatively, local doming occurs due to impingement of electron beams which have high energy when a high luminance image is locally displayed.
Second, the heat of the mask body gradually propagates to the mask frame, and both the mask body and the mask frame thermally expand, as a result of which the color purity lowers.
Lowering of the color purity due to doming of the mask body in the above first manner is caused by displacement of the landing positions of electron beams on the phosphor layers from the predetermined target positions toward the center of the screen. This lowering of color purity (hereinafter referred to as PD-1) can be restricted by forming the mask body by a material having a low thermal expansion coefficient, such as invar, so as to reduce the degree of doming.
In contrast, lowering of the color purity due to thermal expansion of the mask body and the mask frame (which is hereinafter referred to as PD-2) occurs through the following process: the mask body is pulled in a radial direction due to thermal expansion of the mask frame, and as a result, the landing positions of the electron beams are displaced from the predetermined positions on the phosphor layers in a radial direction of the screen. The PD-2 can be restricted, if the mask frame, as well as the mask body, is formed of a material having a low thermal expansion coefficient. However, in general, such a material having a low thermal expansion coefficient is expensive, as compared with a cold-rolled steel plate applied to an ordinary cathode ray tube. Therefore, the material increases the manufacturing cost of the color cathode ray tube, and it is not advisable to use the material. Conventionally, the mask frame is thus formed of a cold-rolled steel plate. Instead, various methods have been proposed, which use various supporting members having specific shapes and formed of specific materials, as the elastic supporting member supporting the shadow mask, in order to prevent the PD-2.
In the above conventional color cathode ray tube, as described above, the structure for supporting the shadow mask is designed such that pushing pressures from the supporting members act equally on the long and short side walls of the mask frame, when the elastic supporting members fixed to the mask frame are engaged with the supporting pins provided at the panel. Thus, the pushing pressure acting on unit volume (unit surface area.times.plate thickness) of the long side walls differs from the pushing pressure acting on unit volume of the short side walls.
In this case, if the elastic supporting members are attached to the mask frame so as to extend in the same rotating direction, the following problem arises: the mask frame is deformed to be lozenge-shaped, and electron beams are incident such that in the vicinity of four diagonal portions of the screen, the beam spots formed on the phosphor layers move non-uniformly in a radial direction of the screen, and as a result, the color quality of the image lowers.
It is difficult to correct lowering of the color purity, which is caused by the above-mentioned non-uniform movement of the beams spots, even if the shape and material of the supporting member are changed as in the proposed various methods.